The present invention relates to fasteners and, more particularly, to a thread-forming screw and its method of manufacture.
It is common practice to use a thread-forming screw to form a female screw thread in a work piece. This avoids the extra step of using a tap to form the mating screw thread prior to application of a screw. An advancement in the fastener art was the introduction of a thread-forming screw with the lead in or distal end portion of the shank having trilobular thread turns. The lobes of each turn of this thread work the material of the wall forming the pilot hole, to form a female thread, while arcuate thread sections between the lobes provide regions of relief to reduce the driving torque needed to form the new thread. For a further description of the structure, operation and method of manufacturing this screw, reference may be made to commonly assigned U.S. Pat. No. 3,461,470.
While the trilobular screw works well, it does have areas susceptible to improvement. This screw does not have a fully formed sharp crest on the first few turns of the thread and has been found to have a tendency to tip upon initial threading. The present invention is directed to forming a self-threading screw that has a reduced driving torque than of fasteners such as shown in U.S. Pat. No. 3,461,470. In that patent, the leading end of he wire is cold headed in a die to form a blank having a tapered distal end. The formation of this taper tends to reduce the life of the blank-forming die because the die must reduce a very substantial cross-sectional area of the length of wire and then eject this tapered end from the die forming it.
A number patents disclose the formation of a conical end or tip on a blank which is then formed into a screw thread or further cold worked. For instance, Veldman U.S. Pat. No. 4,235,149 discloses a blank with a frusto-conical section on its distal end which is rolled by special dies to have concave surfaces formed in the tapered lead-in section between spaced lobes. Between the concave surfaces and the protruding lobes are ramp surfaces smoothly blending the lobes and concavities to each other. However, the first few thread turns of the lead-in section are not fully formed and have flats thereon which require greater turning torque than fully formed threads. U.S. Pat. No. 3,218,656 discloses a conical end on a blank, which conical end is extruded, usually in a cold header, to form four symmetrically arranged and spaced, flattened relief areas. Also, in Reiland U.S. Pat. No. 3,218,905 a tapered conical end is first formed on the blank and then flats are formed in the conical end. The formation of such conical ends on a blank may be eliminated with the present invention.
In U.S. Pat. No. 3,195,156, a circular cross-section wire is drawn through a die to form a trilobular shape for the entire screw including the holding portion which reduces its holding torque power. This extra step of reshaping the entire length of the wire blank and then forming a tapered end on the wire blank is eliminated with the present invention.